In Microsoft Windows:

In Microsoft Windows, you can use the command ipconfig /flushdns to flush the DNS resolver cache.

- Start -> Run -> type cmd
- Hit Enter
- In command prompt, type 

ipconfig /flushdns

- Hit Enter

In Windows 7 and Vista, you will need to do the following:

- Start -> Run -> type cmd
- Hit Ctrl+Shift+Enter (all 3 at the same time)
- Select Continue to run it as administrator
- In command prompt, type 

ipconfig /flushdns

- Hit Enter

In Mac OSX:

In Mac OSX, you can use the command lookupd -flushcache to flush the DNS resolver cache.

bash-2.05a$ lookupd -flushcache

In Max OS X 10.5, please use this command on terminal to flush dns cache:

dscacheutil -flushcache

In Linux:

In Linux, the nscd daemon manages the DNS cache.To flush the DNS cache, restart the nscd daemon. 
To restart the nscd daemon, use the command:

/etc/rc.d/init.d/nscd restart